Mold and manufacturing method thereof, and molded article using the mold

ABSTRACT

The object of the present invention is to provide a mold which has low reactivity with molten alloys and which is inexpensive, a method for manufacturing the same and a molded article using the mold. A mold  40  in accordance with the present invention serves for manufacturing a molded article  60  of a titanium-aluminum alloy or a titanium alloy. At least an initial layer  44   a  of a cavity surface  43  of a mold body  41  constituting the mold  40  is formed of a calcined product of a slurry comprising a filler having cerium oxide as a main component and a binder having silica sol as a main component.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is entitled to the benefit of and incorporated byreference essential subject matter disclosed in International PatentApplication No. PCT/JP2006/317771 filed on Sep. 7, 2006 and JapanesePatent Applications 2005-259218 filed Sep. 7, 2005 and 2005-259219 filedSep. 7, 2005.

TECHNICAL FIELD

The present invention relates to a mold used for manufacturing a moldedarticle of a titanium-aluminum alloy or a titanium alloy, a method formanufacturing the same, and a molded article using the mold.

BACKGROUND ART

Titanium-aluminum alloys composed of titanium aluminide (TiAl), which isan intermetallic compound of Ti and Al have features such as smallweight and high strength. For this reason, titanium-aluminum alloys arepromising materials for turbochargers for automobile engines and rotarymember for gas turbine engines or aircraft jet engines.

Further, because titanium alloys have good corrosion resistance, smallweight and biocompatibility, they have been widely used for automobiles,motorcycles, sports and leisure goods, artificial bones, artificialteeth, and the like.

In order to employ titanium-aluminum alloys or titanium alloys for theaforementioned members, in particular to commercial products, they haveto be molded articles to reduce cost. Molds are required to manufacturemolded articles, and a variety of molds therefor have been suggested(see, for example, Japanese Patent Application Laid-open No. H5-123820,Japanese Patent Application Laid-open No. 2003-225738, Japanese PatentApplication Laid-open No. H 5-277624 and Japanese Patent ApplicationLaid-open No. H 6-292940

DISCLOSURE OF THE INVENTION

However, because titanium alloys have high activity, a needle-likemodified layer (oxygen-rich layer, surface hardened layer) that iscalled α case is sometimes formed in the surface layer portion of theobtained molded articles. The α case has higher hardness and lowermachinability than the α phase of the matrix phase. Therefore, where theα case layer is too thick, a long time is required for chemical millingor mechanical cutting, causing increase in production cost and decreasein productivity.

Further, molds described in Japanese Patent Application Laid-open No.H5-123820, Japanese Patent Application Laid-open No. 2003-225738,Japanese Patent Application Laid-open No. H 5-277624 and Japanese PatentApplication Laid-open No. H 6-292940 above were designed for titaniumalloys, and these molds have been often diverted as molds fortitanium-aluminum alloys.

Because of high activity to titanium-aluminum alloys and titaniumalloys, the reaction of molten alloy and mold has to be taken intoaccount. In particular, because titanium-aluminum alloys have higherreactivity with molds than titanium alloys, it is important to inhibitthis reactivity. This is because, both Ti and Al are active metals, butthe activity of Al is higher than that of Ti, and also becausetitanium-aluminum alloys have a melting point higher than titaniumalloys.

Accordingly, the selection of constituent materials is important formolds designed for titanium-aluminum alloys and titanium alloys. Themold is composed of a ceramic and mainly comprises a filler and a binderfor increasing bonding between the filler particles. Examples ofconstituent materials with low reactivity include zirconium oxide(zirconia), yttrium oxide (yttria), and calcium oxide (calcia) as thefiller, and zirconia sol and organic binders (for example, resins) asthe binder.

However, zirconia and yttria are expensive to be used as the fillers onthe industrial scale. Calcia is difficult to handle because itdecomposes on reaction with water.

Zirconia sol is expensive to be used as the binder on the industrialscale and has low strength at a temperature close to room temperature.Therefore, a separate binder is required to maintain the strength atroom temperature. Further, organic binders are decomposed at a hightemperature, and a separate binder has to be used to maintain thestrength at a high temperature. As a result, the mold cost rises.

It is an object of the present invention, which has been created withthe foregoing in view, to provide an inexpensive mold that has lowreactivity with molten alloys, a method for manufacturing such mold anda molded article using the mold.

The mold in accordance with the present invention that attains theabove-described object is a mold for manufacturing a molded article of atitanium-aluminum alloy or a titanium alloy, wherein at least an initiallayer of a cavity surface of a mold body is formed of a calcined productof a slurry comprising a filler having cerium oxide as a main componentand a binder having silica sol as a main component.

It is preferred that the initial layer and a second layer of the cavitysurface of the mold body be formed from the calcined product of theslurry. Further, the mold is a shell mold or a solid mold.

On the other hand, a method for manufacturing a mold in accordance withthe present invention is for manufacturing a molded article of atitanium-aluminum alloy or a titanium alloy, comprising:

a step of forming an initial layer slurry film by applying an initiallayer slurry comprising a filler having cerium oxide as a main componentand a binder having silica sol as a main component to a surface of a waxmold that is an evaporative pattern mold and by drying thereof;

a step of successively forming slurry films of second and subsequentlayers on a surface of the initial layer slurry film;

a step of forming a mold precursor having a cavity inside the initiallayer slurry film by performing a wax removal treatment with respect tothe wax mold that has been coated with at least two layers of slurryfilms; and

a step of forming a shell mold by performing a calcination treatmentwith respect to the mold precursor to solidify each slurry film.

Here, the step of forming the initial layer slurry film is preferablyrepeated again as a step for forming the second slurry film.

Further, the method for manufacturing a mold in accordance with thepresent invention is for manufacturing a molded article of atitanium-aluminum alloy or a titanium alloy, comprising:

a step of forming a block body of an initial layer slurry by applying aninitial layer slurry comprising a filler having cerium oxide as a maincomponent and a binder having silica sol as a main component to asurface of a wax mold that is an evaporative pattern mold and by dryingthereof,

a step of forming a mold precursor having a cavity inside the block bodyby performing a wax removal treatment with respect to the wax moldhaving the block body; and

a step of forming a solid mold by performing a calcination treatmentwith respect to the mold precursor to solidify the block body.

Further, the method for manufacturing a mold in accordance with thepresent invention is for manufacturing a molded article of atitanium-aluminum alloy or a titanium alloy, comprising:

a step of forming an initial layer slurry film by applying an initiallayer slurry comprising a filler having cerium oxide as a main componentand a binder having silica sol as a main component to a surface of a waxmold that is an evaporative pattern mold and by drying thereof;

a step of forming a block body of the last layer slurry around theinitial layer slurry film;

a step of forming a mold precursor having a cavity inside the initiallayer slurry film by performing a wax removal treatment with respect tothe wax mold that has the initial layer slurry film and the block body;and

a step of forming a solid mold by performing a calcination treatmentwith respect to the mold precursor to solidify the initial layer slurryfilm and the block body.

Here, it is preferred that the step of forming the initial layer slurryfilm be repeated again after the step of forming the initial layerslurry film, and the block body of the last layer slurry be formedaround the initial layer slurry film having a two-layer structure.

The titanium-aluminum alloy molded article in accordance with thepresent invention is formed by casting in use of the above-describedshell mold or solid mold.

Further, the titanium-aluminum alloy molded article in accordance withthe present invention is a titanium alloy molded article that is formedby casting in use of the above-described shell mold or solid mold,wherein a thickness of an α case layer in the surface layer portion ofthe as-cast material is less than 300 μm.

The mold in accordance with the present invention demonstrates anexcellent effect in making it possible to obtain a molded article of atitanium-aluminum alloy with good surface state even as-cast material ora molded article of a titanium alloy with reduced occurrence of α casein the surface layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a wax mold for use in themanufacture of a mold of one preferred embodiment of the presentinvention.

FIG. 2 illustrates a state after a slurry film has been formed aroundthe wax mold in the process for manufacturing the mold of one preferredembodiment of the present invention.

FIG. 3 illustrates the state after wax removal in the process formanufacturing the mold of one preferred embodiment of the presentinvention.

FIG. 4 is a cross sectional view of the mold in one preferred embodimentof the present invention.

FIG. 5 illustrates the state in which a melt is poured into a moldcavity shown in FIG. 4.

FIG. 6 is a cross sectional view of a molded article formed by castingusing the mold shown in FIG. 4.

FIG. 7 is a cross-sectional view of a wax mold used in the manufactureof a mold of another preferred embodiment of the present invention.

FIG. 8 illustrates a state after a slurry film has been formed aroundthe wax mold in the process for manufacturing the mold of anotherpreferred embodiment of the present invention.

FIG. 9 illustrates the formation of a slurry block body around a slurryfilm in the process for manufacturing the mold of another preferredembodiment of the present invention.

FIG. 10 illustrates a state after the slurry block body has been formedin the process for manufacturing the mold of another preferredembodiment of the present invention.

FIG. 11 illustrates a state after the wax has been removed in theprocess for manufacturing the mold of another preferred embodiment ofthe present invention.

FIG. 12 is a cross sectional view of a mold of another preferredembodiment of the present invention.

FIG. 13 illustrates a state in which a melt has been poured into thecavity of the mold shown in FIG. 12.

FIG. 14 is a cross sectional view of a molded article that is formed bycasting using the mold shown in FIG. 12.

FIG. 15 is a planar observation view of a portion of a titanium-aluminumalloy molded article formed by casting by using a mold of one preferredembodiment of the present invention.

FIG. 16 is an enlarged view of the main portion 16 shown in FIG. 15.

FIG. 17 is a planar observation view of a portion of a titanium-aluminumalloy molded article formed by casting by using the conventional mold.

FIG. 18 is an enlarged view of the main portion 18 shown in FIG. 17.

FIG. 19 is a cross-sectional view illustrating an adhesion state of theinitial layer slurry and the wax mold when the filler/binder ratio inthe initial layer slurry is 1.8.

FIG. 20 is a cross-sectional view illustrating an adhesion state of theinitial layer slurry and the wax mold when the filler/binder ratio inthe initial layer slurry is 2.0.

FIG. 21 is a cross-sectional view illustrating an adhesion state of theinitial layer slurry and the wax mold when the filler/binder ratio inthe initial layer slurry is 3.0.

FIG. 22 is a cross-sectional observation view of a titanium alloy moldedarticle formed by casting using a shell mold of one preferred embodimentof the present invention.

FIG. 23 is a cross-sectional observation view of a titanium alloy moldedarticle formed by casting using a conventional shell mold.

FIG. 24 is a cross-sectional observation view of a titanium alloy moldedarticle formed by casting using a solid mold of another preferredembodiment of the present invention.

FIG. 25 is a cross-sectional observation view of a titanium alloy moldedarticle formed by casting using a conventional solid mold.

BEST MODE FOR CARRYING OUT THE INVENTION

Colloidal silica (silica sol) has been used as a binder for castingmolds for Ni-based alloys, Co-based alloys, and Fe-based alloys. Silicasol features chemical stability (low activity), low industrial cost, anda high strength from room temperature to a high temperature. However,silica sol is highly reactive with titanium-aluminum alloys and titaniumalloys. For this reason, silica sol has been conventionally consideredunsuitable for use as a binder for molds for titanium-aluminum alloysand titanium alloys.

However, the results of a keen study performed by the inventorsdemonstrated that by adjusting the constituent materials of a filler fora mold for a titanium-aluminum alloy or a titanium alloy, it is possibleto prevent a vigorous reaction with the titanium-aluminum alloy ortitanium alloy even when silica sol is used as a binder.

One preferred embodiment of the present invention will be describedbelow with reference to the appended drawings.

A cross sectional view of a mold of one preferred embodiment of thepresent invention is shown in FIG. 4.

As shown in FIG. 4, in a mold (shell mold) 40 of the present embodiment,at least an initial layer (in FIG. 4, two layers, that is, an initiallayer 44 a and a second layer 44 b of a cavity surface 43) of a surface(referred to hereinbelow as “cavity surface”) 43 adjacent to a cavity 32of a mold body 41 is formed from a calcined product (referred tohereinbelow as a cerium oxide-silica sol calcined product) of a slurrycomposed of a filler comprising cerium oxide as the main component and abinder comprising silica sol as the main component.

The mold body 41 has a multilayer structure comprising the initial layer(surfacemost layer) 44 a, the second layer 44 b, a third layer 44 c . .. . The slurry calcined product constituting the third layer 44 c andsubsequent layers may be the same as, or different from the ceriumoxide-silica sol calcined product constituting the initial layer 44 aand second layer 44 b. For the third layer 44 c and subsequent layers, acomposition identical to that of the usual mold (for example, a calcinedproduct of a slurry composed of a filler comprising at least oneselected from zirconia, alumina, silica, mullite, zircon, and yttria asthe main component and a binder comprising zirconia sol as the maincomponent) can be applied as the slurry calcined product different fromthe cerium oxide-silica sol calcined product.

The major part, for example, 75 wt. % or more, preferably 80 wt. % ormore of the filler of the initial layer, is cerium oxide, and theremainder is composed of at least one oxide selected from zirconia,alumina, silica mullite, zircon, or yttria. It goes without saying, thatthe filler may be composed only of cerium oxide (filler containing 100wt. % cerium oxide).

Further, the binder for example comprises silica sol (20 to 50% aqueoussolution of silica sol) at 10 to 100 wt. %, preferably 50 to 100 wt. %of the entire binder, the remainder being composed of zirconia sol,yttria sol, alumina sol, or an organic binder.

At least the viscosity of the initial layer slurry is adjusted by thefiller (gram)/binder (gram) ratio to a range of 2-4, preferably 2.5 to3.5. Where the slurry viscosity is low, the slurry does not remain inthe mold (the below-described wax mold 10) and peeling occurs. FIG. 19shows that when the filler/binder ratio is 1.8, peeling occurs, and FIG.20 shows that when the filler/binder ratio is 2.0, peeling is about tostart. As shown in FIG. 21, when the filler/binder ratio is 3.0, peelingdoes not occur and a uniform film is obtained. When the slurry viscosityis too high, the slurry film becomes too thick, a long time is requiredfor drying, and uniform drying is not attained, thereby causing“non-uniformity”. For this reason the filler/binder ratio is taken to beequal to or below 4.0.

In the shell mold 40 of this embodiment, the explanation is conductedwith respect to the case in which the mold body 41 has a three-layerstructure, but the present invention is not limited to suchconfiguration. For example, the mold body 41 can have a two-layerstructure or a structure comprising four or more layers.

In the shell mold 40 of this embodiment, the explanation is conductedwith respect to the case in which the initial layer 44 a and the secondlayer 44 b are composed of the cerium oxide-silica sol calcined product(materials of the same type), but the present invention is not limitedto such configuration. For example, when the thickness of the initiallayer 44 a is sufficiently large (for example, in the case where thethickness of the initial layer 44 a is 500 μm or more), it is preferredthat only the initial layer 44 a be formed from cerium oxide-silica solcalcined product, and that the second and subsequent layers be from thesame material as the usual molds (for example, a calcined product of aslurry composed of a filler comprising zirconium oxide as the maincomponent and a binder comprising zirconia sol as the main component),and with consideration for coating workability, a slurry may be used inwhich the filler/binder ratio is decreased and viscosity is reduced bycomparison with those of the initial layer.

A method for manufacturing the mold of the present embodiment will beexplained below with reference to the appended drawings.

First, as shown in FIG. 1, a wax mold 10 of the same shape and size as atarget precision molded article (see the below-described FIG. 6) isprepared in advance.

Then, an initial layer slurry is coated around the wax mold 10, then theinitial layer stucco is coated, and then dried, to form an initial layerslurry film 24 a, as shown in FIG. 2. Then, a second layer slurry iscoated around the initial layer slurry film 24 a, the second layerstucco is coated, and then dried, to form the second layer slurry film24 b. The initial layer slurry and the second layer slurry areidentical. In other words, the initial layer slurry film 24 a and thesecond layer slurry film 24 b constitute a two-layer structure of theinitial layer slurry film 24 a.

Here, the initial layer slurry and the second layer slurry are prepared,for example, by mixing 1 kg of a binder comprising silica sol as themain component with 2 to 4 kg of a filler comprising cerium oxide as themain component. For example, at least one compound selected fromzirconia, alumina, silica, mullite, and yttria of about #60 to 160 meshcan be used as the stucco (refractory particles that are scattered over,and caused to adhere to the slurry surface) of the initial layer andsecond layer, but no specific limitation is placed on the particle sizeand material thereof. A dipping method, a blowing method, and a coatingmethod can be used for applying the slurry, but the dipping method ispreferred.

A third layer slurry is then coated around the second layer slurry film24 b, then the third layer stucco is coated, and then dried to form athird layer slurry film 24 c. Here, the steps of forming the slurryfilms of the third and subsequent layers are performed appropriately andrepeatedly as necessary. As a result, the thickness of the entire slurryfilm is controlled to the desired thickness. No specific limitation isplaced on the third layer slurry and the slurry of subsequent layers,and also on the constituent materials of the third layer stucco and thestucco of subsequent layers, and any slurry and stucco that have beenusually used for shell molds can be applied.

The wax of the wax pattern 10 is then removed using steam, whereby amold precursor 30 is obtained, as shown in FIG. 3. The mold precursor 30has a cavity 32 inside the precursor body 31 configured of three layersof slurry films 24 a, 24 b, 24 c.

The mold (shell mold) 40 of the present embodiment is then obtained, asshown in FIG. 4, by subjecting the mold precursor 30 to calcinationtreatment. The shell mold 40 has the cavity 32 inside the mold body 41configured of three layers: initial layer 44 a, second layer 44 b, andthird layer 44 c.

Then, as shown in FIG. 5, a melt 50 of a titanium-aluminum alloy ortitanium alloy is poured into the cavity 32 of the shell mold 40 andcasting is performed. The shell mold 40 is then cooled to solidify themelt 50, thereby completing the casting process. As a result, a castbody is formed inside the shell mold 40.

The shell mold 40 is then dipped into a high-temperature alkali bath orthe like, the shell, that is, the mold body 41 is dissolved and removed,and knockout is performed to obtain a molded article 60 of atitanium-aluminum alloy or a titanium alloy, as shown in FIG. 6. Aphysical method (for example, blast cleaning) can used for the knockout.Sandblasting, shot blasting, or water jet (blowing of high-pressurewater) may be used for blast cleaning. Shakeout may be also used as aphysical method other than blast cleaning.

The effect of the mold 40 of the present embodiment will be explainedbelow.

In the mold (shell mold) 40 of the present embodiment, the initial layer44 a and the second layer 44 b of the cavity surface 43 of the mold body41 that comes into direct contact with the melt 50 of titanium-aluminumalloy is formed from the cerium oxide-silica sol calcined product.

Here, cerium oxide used as the main component of the filler of the shellmold 40 is hardly a stable oxide in comparison with zirconia or yttria.This is also clear from the comparison of free energies.

However, cerium oxide demonstrates excellent stability with respect toTi and neither reacts directly with Ti nor is reduced by the melt 50 oftitanium-aluminum alloy poured into the cavity 32 of the shell mold 40.The inventors noticed this specific feature of cerium oxide. Thus, byusing cerium oxide as the main component of the filler of the mold body41, it is possible to prevent the melt 50 of titanium-aluminum alloyfrom reacting with the mold body 41 and oxidizing inside the cavity 32.

Further, silica sol that is used as the main component of the binder ofthe mold body 41 usually reacts vigorously with the melt 50 oftitanium-aluminum alloy. However, by using cerium oxide as the maincomponent of the filler, as in the shell mold 40 of the presentembodiment, it is possible to prevent a vigorous reaction between silicasol and titanium-aluminum alloy even when silica sol is used for thebinder. Further, because silica sol is chemically stable (low activity),industrially inexpensive, and has a high strength within a range fromroom temperature to a high temperature, when silica sol is used, thestrength is maintained with single sol and it is not necessary to useother sols or organic binders for the binder.

Further, because cerium oxide is less expensive than zirconia or yttria,using cerium oxide as the main component of the filler of the shell mold40 makes it possible to reduce the cost of materials for the mold. Onthe other hand, because silica sol has been used as a binder for theusual molds (for example, molds for Ni-based alloys), by using silicasol as the main component of the binder for the shell mold 40, it ispossible to expect a significant cost reduction since the binder can beshared. Because of these factors, an inexpensive shell mold 40 can beobtained.

By forming the initial layer 44 a and second layer 44 b of the cavitysurface 43 of the mold body 41 from the cerium oxide-silica sol calcinedproduct, it is possible to suppress reliably (or almost reliably) theoxidation of titanium-aluminum alloy and the reaction between silica soland titanium-aluminum alloy, to suppress the formation of a layercontaining a large amount of oxygen on the surface of the molded article60, and to inhibit the adhesion (baking) of the mold body 41 to thesurface of the molded article 60.

For example, as shown in FIG. 15 and FIG. 16, in the titanium-aluminumalloy molded article 150 that was formed by casting using the shell mold40 of the present embodiment, practically no baking of the mold body tothe surface of the molded article was observed. Thus, thetitanium-aluminum alloy molded article 150 had beautiful surfaceappearance and a smooth surface. By contrast, as shown in FIG. 17 andFIG. 18, in the titanium-aluminum alloy molded article 160 formed bycasting using zirconia as the main component of the filler and silicasol as the main component of the binder of the mold, a large amount ofbaked material 161 of the mold body appeared on the surface of themolded article. Thus, the titanium-aluminum alloy molded article 160 hadpoor surface appearance, a rough surface, and poor surface state.

Thus, in the titanium-aluminum alloy molded article 150 formed bycasting using the shell mold 40 of the present embodiment, practicallyno mold body is baked to the molded article surface. Therefore, goodsurface state can be obtained by simple blast cleaning. For example, thetitanium-aluminum alloy molded article 150 has an average surfaceroughness of an as-molded article of 200 μm or less, preferably 50 μm orless. Therefore, even the as-cast titanium-aluminum alloy molded article150 has a sufficiently good surface state and does not require surfacefinishing treatment such as chemical milling or mechanical cutting (orrequired only very small surface finishing). Therefore, thetitanium-aluminum alloy molded article 150 makes it possible to reducethe number of production steps, reduce the production cost, and improveproductivity in comparison with the conventional titanium-aluminum alloymolded article 160.

The shell mold 40 of the present embodiment can be manufactured bychanging the formation steps of the initial layer slurry film 24 a andthe second layer slurry film 24 b (or only the initial layer slurry film24 a). Therefore, the shell mold 40 of the present embodiment can bemanufactured without substantial changes in the already existingproduction line for the conventional shell mold and, as a consequence,the increase in production cost can be suppressed.

By performing casting using the melt 50 of a titanium alloy in the shellmold 40 of the present embodiment, it is possible to inhibit theoccurrence of a hardened layer (α case) comprising a large amount ofoxygen in the surface layer portion of the molded article 60. Thethickness of the α case layer occurring in the surface layer portion ofthe obtained molded article 60 is thin, which is less than 300 μm,preferably less than 250 μm.

For example, as shown in FIG. 22, in a titanium alloy molded article 220formed by casting using the shell mold 40 of the present embodiment, thethickness of the α case layer occurring in the surface layer portion wasabout 220 μm. By contrast, as shown in FIG. 23, in the titanium alloymolded article 230 formed by casting using zirconia as the maincomponent of the filler and silica sol as the main component of thebinder of the mold, the thickness of the α case layer occurring in thesurface layer portion was about 500 μm. Therefore, it is clear that thethickness of the α case layer in the titanium alloy molded article 220is less than half that in the titanium alloy molded article 230.

Thus, in the titanium alloy molded article 220 formed by casting usingthe shell mold 40 of the present embodiment, the occurrence of the αcase layer in the surface layer portion is reduced. Therefore, a timerequired for surface treatment (chemical milling, mechanical cutting, orthe like) is shortened in comparison with that for the conventionaltitanium alloy molded article 230. Accordingly, productivity of thetitanium alloy molded article 220 is increased and the production costof the titanium alloy molded article 220 can be reduced. Further,because no significant surface treatment is required for the titaniumalloy molded article 220 to obtain the final product and the differencein dimensions between the titanium alloy molded article 220 and thefinal product is small, the material yield is good and the material costof the titanium alloy molded article 220 can be reduced.

The mold 40 of the present embodiment is suitable as a mold forprecision molded articles. Examples of titanium-aluminum alloy precisionmolded articles include rotary members for turbochargers for automobileengines, gas turbine engines, and aircraft jet engines, and alsoheat-resistant tools. Examples of titanium alloy precision moldedarticles include automobile and motorcycle parts, sports and leisurearticles, artificial bones, artificial teeth, and heat exchangers.

Another embodiment of the present invention will be described below withreference to the appended drawings.

A cross sectional view of the mold of another preferred embodiment ofthe present invention is shown in FIG. 12.

As shown in FIG. 12, in the mold (solid mold) 120 of the presentembodiment, at least the initial layer (in FIG. 12, two layers: aninitial layer 44 a and a second layer 44 b of a cavity surface 123) ofthe cavity surface 123 of a mold body 121 is formed from a ceriumoxide-silica sol calcined product.

The mold body 121 is configured by a block-shaped body portion 124 and alayer portion 125 adjacent to a cavity 112. The layer portion 125 has atwo-layer structure comprising the initial layer 44 a and the secondlayer 44 b. The slurry calcined product constituting the body portion124 may be same as, or different from the cerium oxide-silica solcalcined product constituting the initial layer 44 a and the secondlayer 44 b. For the body portion 124, a composition identical to that ofthe usual mold (for example, a calcined product of a slurry composed ofa filler comprising at least one selected from zirconia, alumina,silica, mullite, zircon, and yttria as the main component and a bindercomprising zirconia sol as the main component) can be applied as theslurry calcined product different from the cerium oxide-silica solcalcined product.

The cavity surface 123 of the mold body 121 in the shell mold 120 of thepresent embodiment may also have a one-layer structure or a structurecomprising three or more layers.

When the thickness of the initial layer 44 a in the solid mold 120 ofthe present embodiment is sufficiently large, only the initial layer 44a may be formed from the cerium oxide-silica sol calcined product, andthe second layer 44 b may be from the same material as the body portion124.

A method for manufacturing the mold of the present embodiment will beexplained below with reference to FIG. 7 to FIG. 14. Componentsidentical to those shown in FIG. 1 to FIG. 6 are denoted by identicalreference numerals and the explanation of these components is omitted.

First, as shown in FIG. 7, a wax mold of the same shape and size as atarget precision case article (see the below-described FIG. 14) isprepared in advance.

Then, an initial layer slurry is coated around the wax mold 70, then theinitial layer stucco is coated, and then dried, to form an initial layerslurry film 24 a, as shown in FIG. 8. Then, a second layer slurry iscoated around the initial layer slurry film 24 a, the second layerstucco is coated, and then dried, to form the second layer slurry film24 b. The initial layer slurry and the second layer slurry areidentical. In other words, the initial layer slurry film 24 a and thesecond layer slurry film 24 b constitute a two-layer structure of theinitial layer slurry film 24 a.

Then, as shown in FIG. 9, the wax mold 70 provided with the slurry films24 a, 24 b is disposed in a space 92 of a mold box 91, and a last layerslurry 93 is injected into the space 92. The last layer slurry 93 is ofa type that cures naturally with the passage of time and mayappropriately contain an organic compound (for example, a phenolicresin), a curing agent, and a refractory material. Natural curing of thelast layer slurry 93 forms, as shown in FIG. 10, a block body 103 of thelast layer slurry 93 around the wax mold 70 provided with the slurrylayers 24 a, 24 b.

The wax of the wax mold 70 is then removed using steam and a moldprecursor 110 is obtained, as shown in FIG. 11. The mold precursor 110has a cavity 112 inside a precursor body 111. The precursor body 111 iscomposed of a block body 103, which is a body portion, and slurry films24 a, 24 b adjacent to the cavity 112.

A mold (solid mold) 120 of the present embodiment is then obtained, asshown in FIG. 12, by performing calcination treatment of the moldprecursor 110.

The solid mold 120 has the cavity 112 inside the mold body 121 composedof the body portion 124 and the layer portion 125.

Then, as shown in FIG. 13, the melt 50 of a titanium-aluminum alloy or atitanium alloy is poured into the cavity 112 of the solid mold 120 andcasting is performed. The solid mold 120 is thereafter cooled tosolidify the melt 50, and the casting process is completed. As a result,a cast body is formed inside the solid mold 120.

Then, as shown in FIG. 14, a molded article 140 of titanium-aluminumalloy or titanium alloy is obtained by removing the cast body from thesolid mold 120.

In the method for manufacturing the mold 120 of the present embodiment,the case is explained in which the block body 103 of a last layer slurry93 is formed around the slurry films 24 a, 24 b of a two-layerstructure, but this configuration is not limiting. For example, theblock body may be directly formed around the wax mold 70 in onemanufacturing process. Thus, it is possible to dispose the wax mold 70inside the space 92 of the mold box 91, then pour the last layer slurry93 into the space 92, and form the block body composed of only the lastlayer slurry 93 directly around the wax mold 70. The last layer slurry93 is identical to the initial layer slurry.

The effect obtained with the mold 120 of the present embodiment isidentical to that obtained with the mold 40 of the above-describedembodiment.

By performing casting using the melt 50 of a titanium alloy in the solidmold 120 of the present embodiment, it is possible to inhibit theoccurrence of a hardened layer (α case) comprising a large amount ofoxygen in the surface layer portion of the molded article 140, and thethickness of the α case layer is thin, which is less than 300 μm. Forexample, as shown in FIG. 24, in a titanium alloy molded article 240formed by casting using the solid mold 120 of the present embodiment,the thickness of the α case layer occurring in the surface layer portionwas about 280 μm. By contrast, as shown in FIG. 25, in the titaniumalloy molded article 250 formed by casting using zirconia as the maincomponent of the filler and silica sol as the main component of thebinder of the mold, the thickness of the α case layer occurring in thesurface layer portion was about 500 μm. Therefore, it is clear that thethickness of the α case layer in the titanium alloy molded article 240is about half that in the titanium alloy molded article 250.

The mold 120 of the present embodiment is suitable as a mold forultralarge molded articles, decorative articles, artificial teeth, andartificial bones than the molds for precision molded articles. Becausethe mold 120 has high endurance and small number of layers, itsimplifies the manufacturing steps and, therefore, demonstratesexcellent cost performance.

It goes without saying that the present invention is not limited to theabove-described embodiments and may be modified in a variety of ways.

1. A mold for manufacturing a molded article of a titanium-aluminumalloy or a titanium alloy, wherein at least an initial layer of a cavitysurface of a mold body is formed of a calcined product of a slurrycomprising a filler having cerium oxide as a main component and a binderhaving silica sol as a main component.
 2. The mold according to claim 1,wherein the initial layer and a second layer of the cavity surface ofthe mold body are formed from the calcined product of the slurry.
 3. Themold according to claim 1, wherein the mold is a shell mold.
 4. The moldaccording to claim 1, wherein the mold is a solid mold.
 5. A method formanufacturing a mold for manufacturing a molded article of atitanium-aluminum alloy or a titanium alloy, comprising: a step offorming an initial layer slurry film by applying an initial layer slurrycomprising a filler having cerium oxide as a main component and a binderhaving silica sol as a main component to a surface of a wax mold that isan evaporative pattern mold, and by drying thereof; a step ofsuccessively forming slurry films of second and subsequent layers on asurface of the initial layer slurry film; a step of forming a moldprecursor having a cavity inside the initial layer slurry film byperforming a wax removal treatment with respect to the wax mold that hasbeen coated with at least two slurry films; and a step of forming ashell mold by performing a calcination treatment with respect to themold precursor to solidify each slurry film.
 6. The method formanufacturing a mold according to claim 5, wherein the step of formingthe initial layer slurry film is repeated again as a step for formingthe second slurry film.
 7. A method for manufacturing a mold formanufacturing a molded article of a titanium-aluminum alloy or atitanium alloy, comprising: a step of forming a block body of an initiallayer slurry by applying the initial layer slurry comprising a fillerhaving cerium oxide as a main component and a binder having silica solas a main component to a surface of a wax mold that is an evaporativepattern mold, and by drying thereof; a step of forming a mold precursorhaving a cavity inside the block body by performing a wax removaltreatment with respect to the wax mold having the block body; and a stepof forming a solid mold by performing a calcination treatment withrespect to the mold precursor to solidify the block body.
 8. A methodfor manufacturing a mold for manufacturing a molded article of atitanium-aluminum alloy or a titanium alloy, comprising: a step offorming an initial layer slurry film by applying an initial layer slurrycomprising a filler having cerium oxide as a main component and a binderhaving silica sol as a main component to a surface of a wax mold that isan evaporative pattern mold, and by drying; a step of forming a blockbody of a last layer slurry around the initial layer slurry film; a stepof forming a mold precursor having a cavity inside the initial layerslurry film by performing a wax removal treatment with respect to thewax mold that has the initial layer slurry film and the block body; anda step of forming a solid mold by performing a calcination treatmentwith respect to the mold precursor to solidify the initial layer slurryfilm and the block body.
 9. The method for manufacturing a moldaccording to claim 8, wherein the step of forming the initial layerslurry film is repeated again after the step of forming the initiallayer slurry film to form the block body of the last layer slurry aroundthe initial layer slurry film having a two-layer structure.
 10. Atitanium-aluminum alloy molded article that is formed by casting in useof the mold according to claim
 3. 11. A titanium alloy molded articlethat is formed by casting in use of the mold according to claim 3,wherein a thickness of an α case layer in the surface layer portion ofthe as-cast material is less than 300 μm.